1. Field of the Invention
The present invention relates to a dielectric filter for use in a mobile communication device such as a cellular telephone or other portable telephone.
2. Description of the Related Art
The structure of a prior art dielectric filter comprising a dielectric block is shown in FIG. 14. In the following figures, the shaded portions indicate visible portions of the dielectric material of the dielectric block. On these visible portions, no conductor is formed.
As shown in FIG. 14, this dielectric filter has, for example, two resonator holes 2 extending between a pair of opposite end surfaces of the dielectric filter, indicated by reference numeral 1. Inner conductors 3 are formed on the inner surfaces of the resonator holes 2. An outer conductor 4 is formed on the outer surface of the block 1. A pair of input/output electrodes 7 are formed at desired locations on the outer surface of the dielectric block. No inner conductors 3 are not formed at portions (hereinafter referred to as nonconductive portions) close to one end surface 1a (hereinafter referred to as the open end surface) of the openings of the resonator holes 2. These nonconductive portions are isolated from the outer conductor 4. At the opposite surface 1b (hereinafter referred to as the shorted end surface), the nonconductive portions are electrically connected or shorted to the outer conductor 4. This dielectric filter consists of two resonators stages each of which is formed in one of the resonator holes 2. These resonators are interconnected in a so-called comb-line connection (coupling) by stray capacitance created in the nonconductive portions.
In this structure, an external coupling capacitance Ce is produced between each input/output electrode 7 and the corresponding inner conductor 3, as shown in FIG. 14. This external coupling capacitance Ce provides external coupling.
When an antenna filter is constructed by using two such dielectric filters, a phase-adjusting circuit is inserted between one end of each filter, and an antenna terminal acting as the common input/output to and from both filters, so that the phase of reflected waves in the passband of one filter will cause the opposite filter to appear as an open circuit. A lumped constant device such as a capacitive device or an inductive device or a distributed constant line such as a cable or stripline is used as the phrase-adjusting circuit.
In the above-described prior art filter which makes use of the external coupling capacitance Ce to obtain external coupling, if a wide passband or strong external coupling is needed, the area of the input/output electrodes may be increased. Alternatively, the resonator holes may be positioned in eccentric positions to shorten the distance between each input/output electrode and the corresponding inner conductor. In this way, adequate external coupling is derived.
However, the foregoing technique requires use of input/output electrodes having a different shape or different dimensions, whenever a desired external coupling is to be obtained. This makes it difficult to standardize the input/output electrodes.
Furthermore, when the area of the input/output electrodes is increased or the resonator holes are positioned in eccentric locations, the unloaded Q (or, Qo) of each resonator drops. In addition, an increase in the area of the input/output electrodes reduces the effective dielectric constant, thus increasing the resonator's electrical length.
Moreover, when an antenna filter or the like is made, using the prior art dielectric filters as described above, phase-adjusting components such as capacitors, coils, or striplines are required in addition to the dielectric filters. Additionally, an operation for mounting and soldering them to a substrate or for forming them on a substrate is required. Consequently, it is difficult to miniaturize the antenna filters. Hence, the cost of the components or fabrication cost is increased.
In particular, in the prior art dielectric filter, once the degree of external coupling at the input and output portions is determined, its phase is also determined. This makes it impossible to set external coupling and phase independently In consequence, it is difficult to obtain a desired external coupling and a desired phase simultaneously. Where a desired phase is associated with a connection to another filter or external circuit, it is necessary to add a separate part for adjusting the phase.